Consistency of the placebo effect

Objective: The existence of reliable personality predictors of the placebo effect is controversial. For prediction to be possible, the response to placebo must be reliable. We tested the consistency of the placebo effect by assessing the response to four trials of placebo analgesic treatment. Methods: Two identical experimental pain stimuli were administered simultaneously to matching fingers on both hands. Pain sensation was compared between one finger, which was treated with a placebo cream and the other which was not treated. Two placebo creams were used, each with a different label. The procedure was repeated between 1 and 8 days later using the same creams and order of presentation. Two personality traits (acquiescence and absorption) and response expectancy were assessed as potential predictors of the placebo effect. Results: Placebo effects across trials were highly correlated (r=.60 and.77) when placebos bore the same name but were not significantly correlated when placebos had different names. Placebo effects were significantly associated with response expectancy but not with acquiescence or absorption. Conclusions: Context-specific predictions of placebo response (e.g., expectancy) are possible, but personality predictors will not be consistent across contexts. (c) 2008 Elsevier Inc. All rights reserved.